The development of modern semiconductor electronics has followed Moore's law [G. E. Moore, Electronics 38, 114 (1965)] for several decades, with the integration density doubling approximately every two years to give rise to ever more powerful and yet cheaper logic and memory devices. However, as the device minimum feature size shrinks towards fundamental physical limits, which will eventually preclude or slow down scaling to even smaller sizes, there are increasing efforts to search for alternatives to conventional electronic devices. In particular, conventional microelectronic devices use the electron's charge or the flow of electron charge to build useful memory and logic devices. The electron has another fundamental property, its spin, which is a quantum mechanical property that gives rise to magnetism. The spin of an electron has two flavors and is characterized as either being up or down. Electron spin-based microelectronics, or what is often referred to as spintronics, is emerging as a promising technology to rival or replace charge-based electronics. In contrast to conventional electronics, where the electron charge in semiconducting materials is used for device operations, spintronics attempts to harness the electron's spin to process and store information. It is widely believed that spintronics has the potential to bring about a new generation of electronic devices with high speed and density, non-volatility and low power consumption [S. A. Wolf et al., Science 294, 1488 (2001)].
Spintronics takes advantage of the long relaxation time (>100 ns) [J. M. Kikkawa and D. D. Awschalom, Physical Review Letters 80, 4313 (1998)] and large coherence length (>100 μm) [J. M. Kikkawa and D. D. Awschalom, Nature 397, 139 (1999)] of electron spins within semiconducting materials. However, a major obstacle for semiconductor spintronics is the electrical generation of highly spin-polarized carriers within semiconductors, which is often referred to as spin injection. Once spin-polarized electrons or holes (vacancies of electrons in the valence band) are injected they can then be subjected to further spin manipulation and spin detection to create devices with new functionality. In III-V bulk semiconductors such as GaAs, the hole spin relaxation is much faster than the electron spin relaxation due to the strong spin-orbit interaction within the valence sub-bands. Consequently, the hole spin relaxation time is on the scale of the momentum relaxation time (˜100 fs), whereas the electron spin relaxation time can be much longer than the momentum relaxation time. In GaAs based quantum well structures, however, the splitting of the valence sub-bands results in a significant enhancement of the hole spin relaxation time. Hole relaxation times as long as 1 ns have been observed by Roussignol et al. in an n-modulation doped 7.5 nm thick GaAs/AlxGa1-xAs quantum well [Physical Review B 46, 7292 (1992)]. It is anticipated that electron spin relaxation times should be even longer in doped silicon semiconducting materials because of weaker spin-orbit coupling.
Various methods have been adopted to inject electron and hole spins into semiconductors. The very first approaches simply attached ferromagnetic metal contacts to the surfaces of semiconductors and passed electrical current from the metal contact into the semiconductor. Since it is well known that electrical current in ferromagnetic metals is usually dominated by either the spin-up or spin-down electrons, it was supposed that one could use such contacts to directly inject spin-polarized current into semiconductors. However, despite considerable effort over many years the efficiency of spin injection from ferromagnetic metals into semiconductors through diffusive contacts has been determined experimentally to be very low [see, for example, Filip et al., Physical Review B 62, 9996 (2000)]. While for a long time this was regarded as a problem of spin relaxation within the ferromagnet/semiconductor contact region, perhaps due to the poor structural integrity of such contacts, it is now believed that the injection efficiency is fundamentally limited by the mismatch in conductivity between typical ferromagnetic metals and semiconductors [Schmidt et al., Physical Review B 62, R4790 (2000)].
One potential way around the conductivity-mismatch problem is to use ferromagnetic contacts with lower electrical conductivities, such as magnetic semiconductors. In 1999, two groups demonstrated spin injection from two different dilute magnetic semiconducting materials into GaAs based semiconducting heterostructures. Both groups used GaAs-based quantum well (QW) light emitting diode (LED) structures to measure the spin polarization of the injected electrical current. Injected electrons (holes) are combined with holes (electrons) within the QW LED to emit photons. The circular polarization of emitted light is indicative of the spin-polarization of the injected electrons or holes. Fiederling et al. [Nature 402, 787 (1999)] used a Mn and Be doped ZnSe alloy, BeMnZnSe, as a spin-injector for n-doped AlGaAs. BeMnZnSe is paramagnetic but has a very large g-factor, so that by applying large magnetic fields (several Tesla) the electronic levels are Zeeman split such that the lowest energy conduction band states become spin-polarized. Fiederling et al. showed a significant degree of spin polarization of the injected electrons but only at very low temperatures (well below 30K) and in large magnetic fields. Ohno et al. [Nature 402, 790 (1999)] used Mn doped GaAs (GaMnAs) for spin injection into undoped GaAs but only found evidence for very low spin polarization of the injected holes. GaMnAs is believed to be ferromagnetic for low concentrations of Mn dopants but only at low temperatures. The Curie temperature of GaMnAs is below ˜150K. Thus, neither of these dilute magnetic semiconductor spin injectors is useful for practical devices since they only operate at low temperatures.
Theoretical work by Rashba [Physical Review B 62, R16267 (2000)] proposed that the presence of a tunnel barrier between ferromagnetic metals and semiconductors could overcome the conductivity mismatch problem, potentially allowing ferromagnetic metals to be used as spin injectors. Ferromagnetic metals are known to have Curie temperatures much higher than room temperature, making them useful for device applications. Hanbicki et al. [Applied Physics Letters 82, 4092 (2003)] utilized an Fe/GaAs Schottky tunnel barrier to realize spin injection. A Schottky tunnel barrier is typically formed when a metal is placed on a semiconductor, and is due to electronic energy band-bending in the semiconductor and the formation of a depletion region in the surface region of the semiconductor. The extent of the depletion region is largely governed by the concentration of charge carriers in this region, which itself is determined by the dopant concentration of the semiconductor. As shown in FIG. 1A, Hanbicki et al. deposited a 10 nm thick Fe layer 32 on an AlGaAs/GaAs quantum well LED structure 11 by molecular beam epitaxy (MBE). Electrons were injected from the Fe layer 32 into the quantum well LED 11 by applying a bias voltage 42 across the entire structure in a perpendicular magnetic field oriented in the direction given by arrow 52, which aligned the magnetic moment in the ferromagnetic Fe layer 32 to be perpendicular to the film plane (i.e., the plane defined by the interface of the LED structure 11 and the Fe layer 32). The injected electrons recombined with holes in the quantum well LED 11 and emitted light 62. According to the optical selection rules [see, for example, “Optical Orientation”, NorthHolland, Amsterdam, 1984, edited by Meier and Zakharchenya], the circular polarization of the light 62 in this geometry is correlated with the spin-polarization of the injected electrons and, therefore, can be used to determine the spin injection efficiency. Hanbicki et al. measured a spin polarization of 32% at 4.5 K using this optical detection technique. The measured spin polarization, however, decreased rapidly at higher temperatures. Furthermore, to grow the Fe layer 32 with MBE is impractical for device fabrication. The direct contact of the Fe layer 32 with the semiconductor LED 11 could cause intermixing between the two and thus compromise the device thermal stability.
As shown in FIG. 1B, Manago and Akinaga [Applied Physics Letters 81, 694 (2002)] used a 2 nm thick Al2O3 tunnel barrier 24′ grown on an AlGaAs/GaAs quantum well LED 11′ for spin injection. A ferromagnetic electrode 32′, consisting of Co, Fe or Ni80Fe20, was deposited on top of the Al2O3 tunnel barrier 24′ and capped with a Au layer 34′. Electrons were injected from the ferromagnetic layer 32′ into the quantum well LED 11′ by applying a bias voltage 42′ across the entire structure in a perpendicular magnetic field whose orientation is given by arrow 52′, with this magnetic field aligning the magnetic moment in the ferromagnetic layer 32′ to be perpendicular to the film plane. The injected electrons recombined with holes in the quantum well LED 11′ and emitted light 62′, whose circular polarization was used to analyze the injected electron spin polarization. Manago and Akinaga only observed a small polarization of ˜1% at room temperature. As shown in FIG. 1C, Motsnyi et al. [Applied Physics Letters 81, 265 (2002)] formed an Al2O3 tunnel barrier 24″ by oxidizing a 1.4 nm thick Al layer grown on an AlGaAs/GaAs layered structure 12″ for spin injection. A ferromagnetic electrode 32″, consisting of 2 nm Co90Fe10 followed by 8 nm Ni80Fe20, was deposited on top of the Al2O3 tunnel barrier 24″ and capped with 5 nm Cu 34″. Electrons were injected from the ferromagnetic layer 32″ into the semiconductor structure 12″ by applying a bias voltage 42″ across the entire structure in an oblique magnetic field whose orientation is given by the arrow 52″. The injected electrons recombined with holes in the semiconductor structure 12″ and emitted light 62″. In this geometry, the circular polarization of the light does not directly reflect the injected electron spin polarization. Motsnyi et al. used the Hanle effect to deduce the spin injection efficiency [“Optical Orientation”, NorthHolland, Amsterdam, 1984, edited by Meier and Zakharchenya]. The directly measured light polarization was only a few percent at 80 K, and the interpretation of the data could be further complicated by other parasitic effects.
Thus none of these prior art spin injectors give significant spin-polarized electrons at room temperature. What is needed for semiconductor spintronic devices is a source of highly spin-polarized electrons operating at room temperature and prepared using practical fabrication techniques.